A variety of circuits have been developed for converting electrical power at one voltage or frequency to another voltage or frequency. For example, a common solid state circuit structure for converting alternating current (AC) power to another frequency (AC to AC power conversion) includes a rectifying bridge which converts the input AC voltage to a DC voltage on a DC link and a solid state inverter which inverts the DC voltage to AC power at a desired frequency. A straightforward method for converting the output voltage level to a different level than that of the input voltage is a step up or step down transformer, either on the input or output side. However, transformers are relatively large, heavy and expensive. In particular, if the power supply circuit is intended to allow the output voltage to be maintained at a desired level despite variations in the input voltage (or conversely, to allow the output voltage to be adjusted to a desired level at a constant input voltage), then more complex transformer structures may be required. For example, some power conditioning circuits utilize tap-changing transformers in which the primary includes a number of separate taps which are connected to the input terminals by static switches, with a control circuit monitoring the output voltage and switching the proper static switches on the input side to maintain approximately the desired voltage on the output side. Tap changing arrangements, however, require many switches, such as thyristors, and only permit coarse control of the output voltage. Furthermore, the distortion of the voltage on the load cannot be well controlled.
Another type of line conditioning structure utilizes a ferroresonant transformer in which the transformer itself is connected in a circuit which provides passive compensation for short term changes in the input voltage. Ferroresonant transformers are widely used and give reasonable voltage regulation, typically plus or minus 4% at the output for minus 20% to plus 10% change in the input voltage. However, output distortion can be sensitive to load input harmonics, and the input currents drawn may have undesirable power factors and distortion levels.
Both the tap changing and the ferroresonant transformers are restricted by the presence of the low frequency transformer which must be able to handle full rated power. For example, for a 1 kilo volt ampere (KVA) line conditioner, the transformer by itself can weigh over thirty to forty pounds, and is the major component determining the size of the cabinet containing the power conditioning apparatus.
The Umezu U.S. Pat. No. 4,656,571 uses a voltage doubler to increase the available AC output voltage. However, such circuits typically require that the input AC voltage be fully converted to a DC voltage level on a DC bus and then inverted to AC power utilizing a full bridge, for example, four switching devices to provide a single phase output voltage and six switching devices to provide a three phase output, as shown in the patent to Umezu.
Static power converters are also commonly used in uninterruptible power systems (UPS) in which the converter operates to supply power to a load from the AC source during normal operation, switching to supply power from a storage device (e.g., a battery) connected across the DC bus lines during line power failure. A simple switching converter which provides UPS operation with only two inverter switches is shown in U.K. patent application GB 2,111,326A to Berman.